Magnetic resonance imaging and spectroscopy at the nanoscale via probe paramagnetic centers

在这个由化学测量和成像项目资助的项目中，纽约城市大学（城市学院）的Carlos a . Meriles正在开发磁共振成像（MRI）技术，用于各种材料，包括活组织，其分辨率比目前可用的要高得多。核磁共振成像在诊断医学中是一项众所周知的技术，但它在可以可视化的体内结构的大小方面受到限制。最近的进展已经使单个细胞内的小细胞器可视化成为可能，但是探测这些细胞器内部的能力仍然困扰着研究人员。该项目旨在改进核磁共振成像，以便在不采用侵入性手段的情况下获得更小物体的清晰图像。研究的重点是开发一种新型的MRI设备探头。探针由一个微小的尖端组成，可以悬浮在被研究的样品上，探针表面涂有一种特殊的金刚石，其中两个相邻的碳原子已经被去除。一个氮原子取代了其中一个碳原子，而另一个留下了空穴。由此产生的缺陷，被称为氮空位，或NV缺陷，通过改变其旋转方式来响应被研究样品的变化。自旋方向的变化可以通过激光照射后从这个尖端发射的光来检测。如果NV中心绕着向上的轴旋转，发出的光就会更强，但如果旋转围绕着向下指向样品的轴旋转，发出的光就会更弱。样品不同部分发出的光的差异可以输入计算机，并转换成比目前的核磁共振成像图像分辨率高得多的图像。有可能有一天，这项技术甚至可以让科学家可视化大型生物分子（如蛋白质）内的单个原子。因此，通过开发在生物科学和医学中广泛适用的工具，这项工作具有广泛的影响。通过在纽约城市学院以及合作机构的主办实验室举办的暑期活动，为贫困学生提供参与这项研究的机会，它正在产生进一步的广泛影响。该项目通过利用金刚石中的氮空位（NV）中心探索纳米级自旋传感的新模式，解决了目前MRI的局限性。该策略不是检测单个自旋，而是专注于NV中心与定位在大约100立方纳米有效体积上的小自旋集合相互作用的情况。基于该小组最近在纳米尺度上对质子自旋噪声的观察，当前的目标是通过设计新的协议来推进基于nv的自旋传感，以增强观测信号的信息内容，并将该技术的适用性扩大到更一般的样本类别。该工作有两个主要推力：(1)第一个推力区域使用近表面NVs探测金刚石表面的模型样品系统，其分子动力学通过诱导受控相变或限制扩散而改变。正在实施各种磁共振方案，以便通过光谱特征暴露样品分子的组成、迁移率和（如果可能的话）结构。(2)第二个主要推力涉及将重点从纳米尺度光谱转移到基于nv托管扫描尖端的几何结构的纳米尺度成像。一套独特的高纯度金刚石纳米柱是通过自上而下的纳米加工生产的，与afm共聚焦系统相结合，以展示具有纳米级空间分辨率的t1加权自旋成像。由于在金刚石基体中存在额外的顺磁缺陷并不一定对传感有害，因此这项工作还有另一个目标，与第一个目标密切相关，即探索在广泛的工程纳米晶体中初始化和控制自旋浴的替代方案。

In this project funded by the Chemical Measurement and Imaging program, Carlos A. Meriles of the City University of New York (City College) is developing magnetic resonance imaging, or MRI, techniques for a variety of materials, including living tissue, with much higher resolution than is currently available. MRI is a well-known technique in diagnostic medicine but it is limited in the size of structures within the body that can be visualized. Recent advances have made it possible to visualize small organelles within individual cells, but the ability to probe within these organelles still eludes researchers. This project seeks to refine MRI in order to acquire sharp images of ever-smaller objects without going to invasive means. The research is focused on the development of a new type of probe for the MRI device. The probe, consisting of a tiny tip that can be hovered over the sample being studied, is coated with a special kind of diamond in which two adjacent carbon atoms have been removed. A nitrogen atom replaces one of these carbons, but the other is left as an empty hole. The resulting defect, known as a nitrogen vacancy, or NV, defect, responds to variations in the sample being studied by changing the way it spins. The change in spin direction can be detected by light that is emitted from this tip after it has been illuminated by a laser. The emitted light is stronger if the NV center spins around an axis pointing up, but it is weaker if the spin is around an axis pointing down toward the sample. The difference in emitted light in different parts of the sample can be fed into a computer and converted into an image that has a much higher resolution than current MRI images. It is possible that one day this technique may even allow scientists to visualize single atoms within large biological molecules such as proteins. This work is, thus, having a broad impact through the development of tools that will find wide applicability in biological science and medicine. It is having a further broad impact through the development of opportunities for underprivileged students to participate in this research through summer activities at the City College of New York as well as in the host laboratories of partner institutions. The project addresses a current limitation of MRI by exploring a new modality of spin sensing at the nanoscale via the use of a nitrogen vacancy (NV) centers in diamond. Rather than detecting single spins, the strategy focuses on the case where the NV center interacts with small ensembles of spins localized over effective volumes of about a hundred cubic nanometers. Building on the group's recent observation of proton spin noise at the nanoscale, the current goal is to advance NV-based spin sensing via new protocols designed to enhance the information content of the observed signals and broaden the technique's applicability to a more general class of samples. The work has two main thrusts: (1) The first thrust area uses near-surface NVs to probe model sample systems on the diamond surface whose molecular dynamics is changed by inducing a controlled phase transition or by restricting diffusion. Various magnetic resonance schemes are being implemented so as to expose the composition, mobility and, if possible, the structure of the sample molecules via spectroscopic signatures. (2) The second main thrust involves a shift in emphasis from nanoscale spectroscopy to nanoscale imaging via a geometry based on an NV-hosting scanning tip. A unique set of high-purity diamond nanopillars produced via top-down nanofabrication is being combined with an AFM-confocal system to demonstrate T1-weighted spin imaging with nanoscale spatial resolution. Since the presence of additional paramagnetic defects in the diamond host is not necessarily detrimental to sensing, the work has another goal, closely related to the first, to explore alternate protocols conceived to initialize and control the spin bath in a broad set of engineered nanocrystals.